Diffuse intrinsic pontine glioma (DIPG) is a tumor that occurs in the brainstem of children (median age 6-7 years) and bears a dismal prognosis.1,2 Current treatment modalities for DIPG are limited since, due to their location, tumors cannot be surgically removed. Also, the efficacy of chemotherapy is limited by the presence of the blood brain barrier (BBB) and the current standard of care, which includes radiation, has severe side effects. The mutations encountered in DIPG have been recently documented.3-7 We developed a genetically engineered DIPG mouse model harboring the prevalent mutations, i.e., ACVR1 G328V (mACVR1) and H3.1K27M (mH3K27). This DIPG model exhibits an intact immune system,8,9 which is ideal for assessing the efficacy of immunotherapies. Our long-term goal is to understand how the mutations encountered in DIPG cells may impact the efficacy of immune-mediated therapeutic approaches. In addition, in collaboration with Dr. O. Becher (Co-I) we now have access to an additional DIPG mouse model that harbors a mutation in H3.3K27 (H3.3K27M) with PDGF-B expression, CDKN2A and TP53 knock down.10-12 Attempts at eliciting a clinically relevant immune response in pediatric brain tumor patients have met with limited success. This could be due to the brain immune privilege, tumor immune evasion and a paucity of dendritic cells (DCs) within the brain parenchyma. In addition, we and others have previously shown the tumor immune microenvironment is highly immune suppressive.13-19 This further hampers the efficacy of immunotherapies. To overcome this, our team developed a combined immunotherapy/conditional cytotoxic gene therapeutic approach that utilizes adenoviral vectors (Ad) expressing Fms-like Tyrosine Kinase 3 ligand (Flt3L) and Thymidine Kinase (TK) delivered into the tumor mass.13-21 We discovered a novel pathway for the activation of an effective anti-brain tumor immune response mediated by the release of a tumor-derived molecule (high mobility group box 1 protein; HMGB1) that triggers the activation of antigen presenting cells within the tumor microenvironment (TME).17,21 We further demonstrated that cytotoxic T cells are required for tumor regression, long-term survival and immunological memory, which elicits elimination of a recurrent tumor without further treatment.13-24 Also, the therapeutic efficacy of the combined Flt3L/TK gene therapy is enhanced when used in combination with immune checkpoint blockade.19 In this revised R21 application, we propose to assess the therapeutic efficacy and the safety of this immune-mediated approach to treat DIPG bearing mice and thereby uncover the mechanisms that mediate tumor regression and long-term survival. We will perform molecular analysis of the tumor transcriptome in response to the proposed therapies, with the aim to uncover novel mechanisms and therapeutic targets. Our preliminary data provide a strong rationale for the pre-clinical efficacy and safety testing of Flt3L/TK-mediated immunotherapy for DIPG in combination with immune checkpoint blockade.